1. Field of the Invention
This invention relates to implantable cardioverter defibrillator (ICD) systems, and particularly to the electrodes and pulse generators thereof. The invention provides optimal materials for constructing pulse generator housings for use as an electrode.
2. Description of the Prior Art
The departure of the heart from normal action to uncoordinated and ineffectual contractions, "fibrillation," can lead to death within minutes unless corrected. One method of treatment to restore the normal heart action involves passing electrical current through the heart muscle. The effectiveness of such treatment is dependent on a number of factors, including the location of the electrodes used to apply the electrical current, the shape of the electrodes, and the magnitude, timing, and waveform of the current. While all these factors are significant, a fundamental problem of all such electrical treatments arises from tile fact that they all require large currents to accomplish defibrillation. And, because the heart muscle typically presents an electrical impedance in tile range of 40 to 100 ohms, signal amplitudes of several hundred volts are required to obtain the necessary current. The requirements for relatively high voltage and several-ampere currents combine to place great importance on efficient, low-resistance electrode arrangements for delivering tile defibrillation signal to the heart. Ideally the electrode would have no resistance itself and would be placed directly against the heart muscle to avoid the voltage drop across the tissue that surrounds the heart.
Various approaches to the optimal electrode, implanted in the body, have been attempted. For example, tile epicardial-patch electrodes comprise conductive and relatively large- surface area elements stitched directly onto the exterior of the heart. While this approach is satisfactory from an electrical standpoint, the attachment of the electrodes requires a major surgical procedure.
Another approach, the transvenous technique, utilizes a conducting filament threaded through an opening in a vein, and into the heart interior. When the filament coils up in a heart chamber, ideally against the chamber wall, a relatively large-area contact to tile cardiac muscle can be made. This approach requires that two such electrodes be used, one in the right-atrium (RA) position or in the nearby superior vena cava (SVC) position, and the other placed at the right-ventricular-apex (RVA) position. Despite the fact that transvenous electrodes can be inserted with a relatively simple surgical procedure, they have a serious shortcoming. Because of the design constraints that permit them to be threaded through the blood vessels, they cannot be depended upon to make adequate contact with the interior wall of the heart, and therefore they sometimes do not direct adequate current through a sufficient portion of the heart-muscle volume to achieve defibrillation.
Another option is to combine a transvenous electrode with a subcutaneous patch (SUB). This approach implants a shallow, just-under-the-skin conductive element of appreciable area on the patient's left side to serve as an electrode, as illustrated in FIG. 2. Since the patch is not directly on the heart, current must pass through the intervening body tissue and fluid to reach the heart. The resistance of the intervening tissue and fluid requires the application of a higher voltage to achieve the desired current through the heart muscle, and the passage of tile current through the intervening material may lead to patient discomfort. Additionally, while the surgical procedure for implanting the subcutaneous patch is relatively minor compared to that required for implantation of electrodes directly against the heart muscle, it still presents some risk to the patient. Although the subcutaneous-patch approach provides the advantage of simpler and less risky surgery, the proximity of a subcutaneous patch to the body's surface leaves the electrode relatively unprotected, and as a result, such electrodes have been subject to flexure and breakage from mishaps, and even from normal body motions.
A final option is to utilize the pulse generator itself as an electrode. Because of the relatively high voltage and substantial currents involved in treatment, the size and weight of an implanted pulse generator (PG) is an important factor in defibrillation. The package or outer shell of the PG is usually a sealed housing made of titanium, selected for its relatively light weight and corrosion resistance. The weight of the PG is normally in excess of 200 grams, or approximately one half pound. The patient abdominal cavity is normally the chosen implantation site for space and comfort reasons. However, implantation of tile PG nearer the heart, for example in the pectoral region, provides the advantage of a more efficient system which in mm allows the size of the PG to be reduced. PG implantation near the heart also permits use of the metallic PG housing as an electrode, also called a "Can". This is, in a sense, a "free" electrode in that the housing is required in any case. Implanting the PG pectorally involves surgery little more invasive than that required to implant a subcutaneous patch. Furthermore, it eliminates the troublesome requirement for tunneling wires under the skin that accompanies the subcutaneous patch. and the PG is also not subject to crumbling and breakage. It is possible to use the PG enclosure as an electrode in combination with electrodes of the prior art, such as the RVA, SVC and subcutaneous-patch (SUB) electrodes. This facilitates the use of sequential defibrillation pulses having different spatial axes, demonstrated in the prior art to reduce the amount of energy needed for defibrillation (i.e. lower defibrillation threshold). Energy consumption is a vital concern since it is directly related to size and therefore also implantability.
Known "active can" electrode designs have been found to be less than optimal due to oxidation of the can material. Insofar as is known, no device has been made or proposed which solves this problem as applicant has.